1. Field of the Invention
The present invention relates to an actuator system having an output shaft which is held in a first position by a spring and upon command an electric motor drives the shaft to a second position against the force of the spring and holds the shaft in the second position until a further command allows the spring to return the shaft to the first position.
2. Description of Co-Pending Application
In a co-pending patent application of Christopher M. Lange entitled xe2x80x9cDrive Circuit And Method For An Electric Actuator With Spring Returnxe2x80x9d Serial Number 08/904,005, Filed Jul. 31, 1997, issued Jun. 19, 2001 as U.S. Pat. No. 6,249,100B1 and assigned to the assignee of the present invention, an actuator system is shown in which an output shaft is positioned by a spring in a first position and upon command, is driven through a gear train to a second position by an electric motor. The circuit includes an input circuit for providing a current large enough to rotate the shaft against the force of the spring. A rotation sensor produces a signal when the motor and shaft are stalled at the second position and a modulation circuit receiving the signal from the rotation sensor operates to reduce the current to the motor so as to hold the stalled shaft and motor at the second position with a minimum of energy usage. In some applications, this circuit is intended for use in high ambient temperature conditions and accordingly, the rotation sensor utilizes a Hall effect device known to be able to withstand high temperatures. The use of a Hall effect device involves some undesirable features because it is undesirably complicated to implement and is slower than desired. The increased current to the motor at the stalled condition produces an increased torque that can over stress the gear train, at least temporarily, and gear damage may result.
The present invention overcomes these problems by noting that the current in the motor increases sharply when the shaft reaches the stalled position. Thus, detecting the sharp rise in current becomes a simple way to detect the stalled condition, and does not involve waiting for a time period as great or greater than the normal period for one complete revolution to determine the condition, as may be required if only a rotation sensor is relied on. The problem of possible gear damage is overcome in the present invention, with the use of a current limiting circuit to prevent the current in the motor from increasing beyond a predetermined limit chosen so that the gear train will not be damaged. This satisfactorily protects the gear train in a high temperature embodiment of the present invention, which uses the Hall effect devices to sense rotation of the output. However, in a low or normal temperature embodiment of the present invention, the Hall effect devices are not needed but, in order to detect a stalled condition, with a current limiter in operation, the voltage to the motor is rapidly stopped and started so that the current repeatedly and rapidly oscillates just below the current limiting value. A counter is employed to count these oscillations and a predetermined number of such oscillations becomes an indication that the motor is stalled.